Large complex structures composed of semiconductor-grade silicon are being sought for purposes such as a wafer tower 10 illustrated orthographically in FIG. 1 and disclosed by Zehavi in U.S. patent application Ser. No. 09/292,491, filed Apr. 15, 1999, now issued as U.S. Pat. No. 6,225,594. Boyle et al. disclose further details of such towers and their fabrication in U.S. patent application Ser. No. 09/608,557, filed Jun. 30, 2000, incorporated herein by reference in its entirety. The tower of the example includes two bases 12 and four legs 14 joined to the bases 12. A plurality of parallel teeth 16 with intervening slots are machined into each of the legs 14 to support a plurality of silicon wafers on the wafer tower 10 during medium or high temperature processing of the wafers, for example, for annealing at high temperatures or thermal chemical vapor deposition at somewhat lower temperatures.
It is desired that the tower and particularly its legs be composed of the same material as the wafers, that is, silicon, and that the silicon be of semiconductor grade, that is, be of very high purity. Semiconductor-grade silicon is available with impurity levels of less than 1 ppm (parts per million atomic), if concentrations of up to 100 ppm of oxygen, nitrogen, and carbon are ignored, and sometimes the impurity levels are less than 1 ppb (parts per billion atomic). The impurity levels of oxygen, nitrogen, and carbon are far less than 1% atomic, whereby silica, silicon nitride, and silicon carbide are excluded from being characterized as semiconductor-grade silicon. Virgin polysilicon is an especially pure form of silicon grown by thermal chemical vapor deposition using one of several forms of silane as the precursor gas. Silicon has a melting temperature of about 1416° C. and remains strong and tough up to nearly that temperature. Thereby, silicon towers can be designed for extended use at high temperatures. The similarity of the materials of the support structure and of the supported workpieces minimizes differential thermal effects and eliminates contamination from non-silicon material. Furthermore, semiconductor-grade silicon with very low impurity levels is widely available at moderate cost in the form of virgin polysilicon. Support fixtures made of high-purity silicon reduce the danger of minute levels of impurities in the support structure diffusing into the semiconductor wafer and degrading its semiconductor characteristics.
Fabricating large complex silicon structures, however, has presented several challenging technical problems, particularly in joining two silicon members. Some type of fusion welding is desired both to maintain the low impurity levels in the joint and to assure that the joint remains joined at the extreme temperatures being contemplated. Zehavi has suggested laser welding. Plasma welding in an inert gas has also been suggested. While these methods have enjoyed some success, the reproducibility of the process and the overall strength of the weldment are still considered deficient.
A fundamental problem is that welding silicon with perhaps a silicon welding rod requires temperatures in excess of silicon's melting point. Tungsten inert gas (TIG) welders and plasma arc welders are well capable of achieving such temperatures at localized areas adjacent to the seam being welded. However, after the local area has been welded and the welding tip is moved further along the seam, the temperature of the region surrounding the already welded spot rapidly decreases. The resultant thermal stresses induced between hot and cold areas of the large silicon workpieces tend to crack the silicon near the welded seam. Although a cracked area does not unacceptably degrade the strength required for a structure supporting light silicon wafers, the cracks introduce a source of particulate contamination and also serve to initiate further fracturing of the assembled structure during repeated thermal cycling.
Ultrasonic welding of silicon solar cells is also known, but this method is not appropriate for the massive silicon bodies required in towers and similar large structures.
Accordingly, it is desired to achieve a method of joining large pieces of silicon that does not crack the silicon. It is further desired to achieve a joining method that can be used with high-purity silicon and does not significantly increase the silicon impurity level.